I've only been shooting trad a bit over a year. I've been shooting with the thumb of my string hand tucked into my palm. I've seen some folks shoot with their thumbs extended away from their hands, so last week I gave it a try. So far, I've only shot about 150 arrows that way. It's too early to say for sure, but it seems like I'm more consistent, if not more accurate. The latter is a whole different topic.
Thumb up can cause a LOT of tension in your hand and forearm. Thumb down lessens and/or eliminates the tension. The less tension the better IMO.
Arne
i shoot with my thumb down tucked behind my ear. Once i switched to that i got alot cleaner release. I was noticing the tension in my hand when I kept my thumb up. I had a tencecy to kind of hold it out and up. moebow is exactly right abot tension.
Never thought about it until seeing this topic but now that I have, my thumb is just floating behind the fingers almost as if it doesn't exist. I guess that would be considered down and definitely tension free.
I tried thumb up once, but I felt like I had to draw the string more out and around my face as opposed to straight back and into my anchor. I feel more comfortable with it just kind of relaxed and between my hand and face.
IMHO, that question has to be answered based on the individual's unique facial geometry. Not everyone's bone structure, i.e. cheek/jaw line is the same. If the thumb is down does it help/hinder settling into your primary anchor? What affect does it have on eye/shaft alignment? I can shoot with the thumb "up", but relaxed and flat against the base of the index finger (think pics you've seen of Fred Bear) and I can't tell that it causes any perceptible increase in hand tension. At one time I shot with it sticking almost straight up as a secondary anchor against my temple..this did cause tension. Having said all that, I now shoot with it down, but relaxed. My second anchor is now the thumb's base in behind my jawbone. this feels natural and for me is repeatable. But to make it work I had to slightly rotate the hand into my face (less parallel to the string). I have a bit of angle to the string fingers, something coaches mostly discourage, but it works for me. Find what works for you.